A secondary battery is an energy storage body with high energy density, that basically consists of a positive electrode/a negative electrode/a separator/electrolyte, that is capable of charging and discharging as chemical energy and electrical energy are reversibly converted, and that is widely used in small size electronic equipment such as mobile phones and notebooks, etc. Recently, its application is rapidly expanding to hybrid electric vehicles (HEV), plug-in EVs, e-bikes and energy storage systems (EES), in order to respond to environmental problems, high oil prices, energy efficiency and storage demands.
In manufacturing and using such a secondary battery, securing safety of the secondary battery is an important task to be resolved. Especially, a separator, that is generally used in electrochemical elements, exhibit extreme thermal shrinkage behavior in situations of high temperature and the like due to its material characteristics and characteristics of manufacturing process, thereby leading to safety problems of internal short-circuit. Recently, an organic-inorganic composite porous separator was proposed (see Korean Patent Application No. 10-2004-0070096), where a porous inorganic coating layer is formed by coating a mixture of inorganic particles and binder resin on a porous material for a secondary battery separator, in order to secure safety of the secondary battery. However, there exists a problem that when stacking an electrode and a separator to form an electrode assembly, due to insufficient interlayer bonding force, there is a high risk of the electrode and the separator being separated from each other, in which case inorganic particles being de-intercalated in the separating process may act as a local defect in an element.
In order to resolve this problem, Korean Patent Application Laid-open No. 10-2006-0116043 discloses a method comprising adding ethanol to a solution, in which a good solvent such as acetone has been dissolved, and then applying the solution on top of a separator, and drying it, so that a porous bonding layer can be obtained by the phase separation effect. The porous bonding layer obtained in this method has advantages of excellent infiltration and low resistance during battery operation, but due to swelling upon injection in the manufacturing process of a battery, coherence with the separator, that is, the mechanical strength decreases, and low cycling characteristics and interlayer mixing with the porous inorganic coating layer occur, blocking the pores formed in the porous inorganic coating layer, thus leading to a problem of deterioration of air permeability of the separator.